Abstract
The SD-bw line, which was produced by recombination of SD-72 with a cn bw stock, shows fairly high SD activity (K>0.9). This line was irradiated with a high dose of X-rays and many chromosome rearrangements were produced. The SD activity was significantly reduced in many of the irradiated lines: in some cases, the original high activity was recovered in a few generations, while, in other cases, the reduction was permanent. The nature of the chromosome rearrangements was determined through recombination tests and cytological observation of the salivary gland chromosomes, and the results compared with the SD activity. The findings from these experiments are as follows:
1. Only chromosome pairing in the SD region corresponding to the small pericentric inversion of SD lines is essential for the normal function of the SD gene.
2. The Y chromosome may possess some function necessary for the SD gene to operate.
3. There may be modifiers of SD activity on the third chromosome and/or the fourth chromosome.
